Systems and Methods for Varying a Sampling Rate of a Signal

ABSTRACT

Methods and systems are provided that include sampling a light intensity signal at different frequencies based on the waveform of the signal to produce a more accurate digitized signal. The light intensity signal is an analog signal proportional to the intensity of light received at a detector of a pulse oximetry system. In one embodiment, the signal may be sampled exponentially during pulse width periods, such that the end of the pulse width periods where the signal reaches a maximum amplitude may be sampled more frequently. The signal may also be exponentially sampled or oversampled during periods when the signal is expected to near a maximum amplitude. Further, the signal may be sampled less frequently during low amplitude periods of the signal, and during dark periods, such that processing power may be conserved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/512,155, entitled “Exponential Sampling of Redand Infrared Signals,” filed Jul. 30, 2009, the disclosure of which ishereby incorporated by reference in its entirety for all purposes.

BACKGROUND

The present disclosure relates generally medical devices and, moreparticularly, to methods of processing sensed physiological signals.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present disclosure,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certainphysiological characteristics of their patients. Accordingly, a widevariety of devices have been developed for monitoring many suchphysiological characteristics. Such devices provide doctors and otherhealthcare personnel with the information they need to provide the bestpossible healthcare for their patients. As a result, such monitoringdevices have become an indispensable part of modern medicine.

One technique for monitoring certain physiological characteristics of apatient is commonly referred to as pulse oximetry, and the devices builtbased upon pulse oximetry techniques are commonly referred to as pulseoximeters. Pulse oximetry may be used to measure various blood flowcharacteristics, such as the blood-oxygen saturation of hemoglobin inarterial blood, the volume of individual blood pulsations supplying thetissue, and/or the rate of blood pulsations corresponding to eachheartbeat of a patient. In fact, the “pulse” in pulse oximetry refers tothe time varying amount of arterial blood in the tissue during eachcardiac cycle.

Pulse oximeters typically utilize a non-invasive sensor that transmitslight through a patient's tissue and that photoelectrically detects theabsorption and/or scattering of the transmitted light in such tissue.One or more of the above physiological characteristics may then becalculated based upon the amount of light absorbed or scattered. Morespecifically, the light passed through the tissue is typically selectedto be of one or more wavelengths that may be absorbed or scattered bythe blood in an amount correlative to the amount of the bloodconstituent present in the blood. The amount of light absorbed and/orscattered may then be used to estimate the amount of blood constituentin the tissue using various algorithms.

The intensity of the detected absorbed and/or scattered light may resultin an analog signal proportional to the intensity of the detected light,which may be sampled to produce a digitized signal. The digitized signalmay be further processed and used to determine the physiologicalcharacteristics. Typically, the analog signal may be sampledperiodically, such that the sampling density is evenly distributed alongthe signal. However, the analog signal increases in amplitude during apulse of emitted light, and generally does not reach the maximumamplitude until the end of a pulse. Thus, the sampling frequency may notsufficiently sample a light intensity signal where the pulse amplitudeis the highest.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the disclosed techniques may become apparent upon readingthe following detailed description and upon reference to the drawings inwhich:

FIG. 1 illustrates a perspective view of a pulse oximeter in accordancewith an embodiment;

FIG. 2 illustrates a simplified block diagram of a pulse oximeter inFIG. 1, according to an embodiment;

FIG. 3 is a graph depicting the waveform of a light intensity signalreceived at a pulse oximeter in FIG. 1, according to an embodiment;

FIG. 4 is a graph depicting periodic sampling of a light intensitysignal;

FIG. 5 is a graph depicting exponential sampling of a light intensitysignal, according to an embodiment;

FIG. 6 is a graph depicting another method of exponential sampling of alight intensity signal, according to an embodiment; and

FIG. 7 is a graph depicting a Nyquist sampling method of a lightintensity signal, according to an embodiment.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present techniques will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

A pulse oximeter emitter may emit one or more lights containing red andinfrared (IR) wavelengths into a tissue, and the light that istransmitted and/or scattered by the tissue may be received at adetector. As red and IR light have different wavelengths, and asoxygenated and deoxygenated hemoglobin in the blood absorb differentwavelengths of light, certain physiological characteristics such asblood-oxygen saturation and pulse rate may be determined based on thered and IR light received from the tissue. Typically, the detector(e.g., a photodiode) in the pulse oximeter may produce a currentproportional to the intensities of the red and IR light received by thedetector. The produced current may be the analog signal (also referredto as the “light intensity signal”) that is sampled to produce thedigitized signal used in further processing and/or calculations todetermine the physiological characteristics. For example, the ratio ofred and IR light transmitted through the tissue may be computed todetermine blood-oxygen saturation in accordance with known techniques.

A pulse oximeter may typically emit red and IR light alternately (i.e.,pulses), and a detector may detect the red and IR light that has beentransmitted and/or scattered by the tissue. With each pulse of red andIR light emitted by the emitter, the intensity of the red and IR lightreceived at the detector may vary through the pulse width period. Due tocertain factors such as opacity of the tissue (optical density of thetissue between the emitter and the detector), and the distance betweenthe emitter and the detector, the amount of emitted light that istransmitted through the tissue may not be fully detected until near theend of the pulse width period. Thus, the light intensity signal,proportional to the intensities of the received red and IR light, maynot reach a maximum amplitude until the near the end of the pulse widthperiod. The maximum amplitude of the light intensity signal may includethe most significant information in producing an accurate digitizedsignal. While light intensity signals are typically sampledperiodically, periodic sampling may not have a high sampling densityduring the most significant time of the pulse width period (i.e., whenthe signal is at or near the maximum amplitude). Further, the samplingdensity at less relevant times, including the dark portions, may behigher than necessary.

The present techniques relate to systems and methods for sampling alight intensity signal based on the waveform of the signal. In one ormore embodiments, the sampling frequency may be higher when the signalis at or approaching a maximum pulse amplitude and lower during lessrelevant times. For example, the pulse period of light intensity signalmay be sampled exponentially, rather than periodically, such that theend of the pulse width period where the signal approaches and reachesthe maximum amplitude may be sampled more frequently. In someembodiments, the sampling frequency may be based on the shape of thelight intensity signal. For example, at a certain time during a pulsewidth period, a pulse oximeter may begin exponential sampling,oversampling, or sampling at the Nyquist frequency of the signal untilthe end of the pulse width period. Furthermore, sampling frequency maybe lower during portions of the pulse width period when the signalamplitude is lower, and during detection periods outside of the pulsewidth periods (i.e., dark periods), thus saving on processing power.

Turning to FIG. 1, a perspective view of a medical device is illustratedin accordance with an embodiment. The medical device may be a pulseoximeter 100. The pulse oximeter 100 may include a monitor 102, such asthose available from Nellcor Puritan Bennett LLC. The monitor 102 may beconfigured to display calculated parameters on a display 104. Asillustrated in FIG. 1, the display 104 may be integrated into themonitor 102. However, the monitor 102 may be configured to provide datavia a port to a display (not shown) that is not integrated with themonitor 102. The display 104 may be configured to display computedphysiological data including, for example, an oxygen saturationpercentage, a pulse rate, and/or a plethysmographic waveform 106. As isknown in the art, the oxygen saturation percentage may be a functionalarterial hemoglobin oxygen saturation measurement in units of percentageSpO₂, while the pulse rate may indicate a patient's pulse rate in beatsper minute. The monitor 102 may also display information related toalarms, monitor settings, and/or signal quality via indicator lights108.

To facilitate user input, the monitor 102 may include a plurality ofcontrol inputs 110. The control inputs 110 may include fixed functionkeys, programmable function keys, and soft keys. Specifically, thecontrol inputs 110 may correspond to soft key icons in the display 104.Pressing control inputs 110 associated with, or adjacent to, an icon inthe display may select a corresponding option. The monitor 102 may alsoinclude a casing 111. The casing 111 may aid in the protection of theinternal elements of the monitor 102 from damage.

The monitor 102 may further include a sensor port 112. The sensor port112 may allow for connection to an external sensor 114, via a cable 115which connects to the sensor port 112. The sensor 114 may be of adisposable or a non-disposable type. Furthermore, the sensor 114 mayobtain readings from a patient, which can be used by the monitor tocalculate certain physiological characteristics such as the blood-oxygensaturation of hemoglobin in arterial blood, the volume of individualblood pulsations supplying the tissue, and/or the rate of bloodpulsations corresponding to each heartbeat of a patient.

Turning to FIG. 2, a simplified block diagram of a pulse oximeter 100 isillustrated in accordance with an embodiment. Specifically, certaincomponents of the sensor 114 and the monitor 102 are illustrated in FIG.2. The sensor 114 may include an emitter 116, a detector 118, and anencoder 120. It should be noted that the emitter 116 may be capable ofemitting at least two wavelengths of light, e.g., RED and infrared (IR)light, into the tissue of a patient 117, where the RED wavelength may bebetween about 600 nanometers (nm) and about 700 nm, and the IRwavelength may be between about 800 nm and about 1000 nm. The emitter116 may include a single emitting device, for example, with two lightemitting diodes (LEDs) or the emitter 116 may include a plurality ofemitting devices with, for example, multiple LED's at various locations.Regardless of the number of emitting devices, the emitter 116 may beused to measure, for example, water fractions, hematocrit, or otherphysiologic parameters of the patient 117. It should be understood that,as used herein, the term “light” may refer to one or more of ultrasound,radio, microwave, millimeter wave, infrared, visible, ultraviolet, gammaray or X-ray electromagnetic radiation, and may also include anywavelength within the radio, microwave, infrared, visible, ultraviolet,or X-ray spectra, and that any suitable wavelength of light may beappropriate for use with the present disclosure.

In one embodiment, the detector 118 may be an array of detector elementsthat may be capable of detecting light at various intensities andwavelengths. In operation, light enters the detector 118 after passingthrough the tissue of the patient 117. The detector 118 may convert thelight at a given intensity, which may be directly related to theabsorbance and/or reflectance of light in the tissue of the patient 117,into an electrical signal. That is, when more light at a certainwavelength is absorbed or reflected, less light of that wavelength istypically received from the tissue by the detector 118. For example, thedetector 118 may comprise one or more photodiodes, or any other elementcapable of converting light into either a current or voltage. Afterconverting the received light to an electrical signal, the detector 118may send the signal to the monitor 102, where physiologicalcharacteristics may be calculated based at least in part on theabsorption of light in the tissue of the patient 117.

Additionally the sensor 114 may include an encoder 120, which maycontain information about the sensor 114, such as what type of sensor itis (e.g., whether the sensor is intended for placement on a forehead ordigit) and the wavelengths of light emitted by the emitter 116. Thisinformation may allow the monitor 102 to select appropriate algorithmsand/or calibration coefficients for calculating the patient's 117physiological characteristics. The encoder 120 may, for instance, be amemory on which one or more of the following information may be storedfor communication to the monitor 102: the type of the sensor 114; thewavelengths of light emitted by the emitter 116; and the propercalibration coefficients and/or algorithms to be used for calculatingthe patient's 117 physiological characteristics. In one embodiment, thedata or signal from the encoder 120 may be decoded by a detector/decoder121 in the monitor 102.

Signals from the detector 118 and the encoder 120 may be transmitted tothe monitor 102. The monitor 102 may include one or more processors 122coupled to an internal bus 124. Also connected to the bus may be a RAMmemory 126 and a display 104. A time processing unit (TPU) 128 mayprovide timing control signals to light drive circuitry 130, whichcontrols when the emitter 116 is activated, and if multiple lightsources are used, the multiplexed timing for the different lightsources. TPU 128 may also control the gating-in of signals from detector118 through a switching circuit 134. These signals are sampled at theproper time, depending at least in part upon which of multiple lightsources is activated, if multiple light sources are used. The receivedsignal from the detector 118 may be passed through an amplifier 136, alow pass filter 138, and an analog-to-digital converter 140 foramplifying, filtering, and digitizing the electrical signals the fromthe sensor 114. The digital data may then be stored in a queued serialmodule (QSM) 142, for later downloading to RAM 126 as QSM 142 fills up.In an embodiment, there may be multiple parallel paths for separateamplifiers, filters, and A/D converters for multiple light wavelengthsor spectra received.

In an embodiment, based at least in part upon the received signalscorresponding to the light received by detector 118, processor 122 maycalculate the oxygen saturation using various algorithms. Thesealgorithms may require coefficients, which may be empirically determinedFor example, algorithms relating to the distance between an emitter 116and various detector elements in a detector 118 may be stored in a ROM144 and accessed and operated according to processor 122 instructions.

In one embodiment, a signal 156 proportional to the intensity of lightreceived by the detector 118 is illustrated in the graph 150 of FIG. 3.As discussed, the detector 118 may be capable of sensing the intensityof light that is emitted by the emitter 116 and transmitted through thetissue of the patient 117. The detector 118 may produce a current, orany other analog signal, that is proportional to the intensity of thereceived light. This analog signal 156, also referred to as a lightintensity signal, may be sampled to produce a digitized signal, whichmay be suitable for further processing and/or calculations. For example,the processor 122 may apply algorithms to the digitized signal todetermine various physiological characteristics.

The light intensity signal 156 may have varying amplitude 152 resultingfrom the one or more lights (e.g., red and IR light) emitted by theemitter 116 throughout the detection time 154 (x-axis of the graph 150).The amplitude 152 (the y-axis of the graph 150) may be proportional to acurrent output by the detector 118 in response to the intensity of lightreceived. As discussed, red light and IR light may be used because theyhave different wavelengths which are absorbed differently by oxygenatedand deoxygenated blood. Such absorption characteristics may be useful indetermining physiological parameters such as oxygen saturation in theblood. However, some detectors 118 may not differentiate between redlight and IR light if both are received simultaneously. To differentiatebetween the absorption and/or transmission of different lights, thepulse oximeter 100 may alternately turn a red light and an IR light onand off (i.e., pulses), such that when the red light is on, the lightreceived at the detector 118 may be processed as red light (i.e., duringthe red pulse width period 158), and when the IR light is on, the lightreceived at the detector 118 may be processed as IR light (i.e., duringthe IR pulse width period 160). Thus, the pulse oximeter 100 may sampleduring the red pulse width period 158 and the IR pulse width period 160to produce a digitized signal which contains information of lightintensity during each pulse width period 158 or 160. Further, the pulseoximeter 100 may also sample during the dark periods 162 and 164 betweenthe pulse width period periods 158 and 160 to filter out DC content suchas ambient light.

The amplitude 152 of the signal 156 may also vary during the red pulsewidth period 158 and the IR pulse width period 160. The signal 156,corresponding to the intensity of transmitted light received at thedetector 118, may not reach a maximum amplitude until near the end ofthe pulse width periods 158 and 160. Variations of the signal 156 duringa pulse width periods 158 and 160 may be based on factors such as theopacity of the tissue (optical density of the tissue between the emitter116 and the detector 118), the distance between the emitter 116 and thedetector 118, etc. Because of these and other factors, the amount ofemitted light that is transmitted through the tissue may not be fullydetected until near the end of the pulse width periods 158 and 160.Thus, the end portions of the pulse width periods 158 and 160 where thesignal 156 approaches a maximum amplitude may contain the most relevantinformation in accurately determining physiological characteristics.

Typically, a pulse oximeter 100 may sample a signal 156 periodically, asdepicted in the graph 170 of FIG. 4, which illustrates the sampling of asignal 156 during a red pulse width period 158. However, as discussed,the most relevant portion 172 of the signal 156 may be the portions ofthe signal 156 approaching or at a maximum amplitude. Thus, a periodicsampling method may result in the even sampling of the signal 156 duringthe most relevant portions 172 and the comparatively less relevantportions 174 of the signal 156. Such a sampling method may result in aless accurate digital signal if, for example, the periodic samplingmethod fails to sample a signal 156 during the maximum amplitude of apulse width period 158. A situation where the maximum amplitude of thesignal 156 during the pulse width period 158 is not sampled may have ahigher probability of occurring when the sampling density (i.e., thenumber of samples in a period of time) is lower.

In one or more embodiments of the present techniques, a signal 156 maybe sampled based on the waveform of the signal 156, such that the signal156 may be sampled more frequently at relevant portions 172 (i.e., theportion where the signal 156 approaches or is at a maximum amplitude),and less frequently at less relevant portions 174 (i.e., other portionsof the signal 156 in the pulse width period 158 before the signal 156nears a maximum amplitude). For example, as illustrated in the graph 180of FIG. 5, the signal 156 may be exponentially sampled during a pulsewidth period 158, such that the relevant portion 172 of the signal 156has a higher sampling density than other portions 174 of the signal 156due to the exponentially increasing sampling frequency within the pulsewidth period 158. One example of an algorithm used in exponentialsampling may be explained in the equation below:

$\begin{matrix}{{f\left( {x,\lambda} \right)} = \begin{matrix}{1 - ^{{- \lambda}\; x}} & {{{for}\mspace{14mu} x} \geq 0} \\0 & {{{for}\mspace{14mu} x} < 0}\end{matrix}} & {{eq}.\mspace{14mu} 1}\end{matrix}$

In eq. 1 above, x may represent the number of samples to be performedduring a pulse width period 158, and λ may be the rate parameter. Insome embodiments, adjusting the rate parameter λ may vary the number ofsamples taken during the pulse width period 158, and may increase thesampling density during the relevant portion 172 of the signal 156. Forexample, λ may have values between 1.0 or 1.5, or may be greater for agreater sampling density during the relevant portion 172 of the signal156 in the pulse width period 158.

In one or more embodiments, the signal 156 may also be sampled with alinearly increasing sampling frequency within the pulse width period158, or sampled with a sampling frequency that is increasedsubstantially in proportion to an expected increase of the signal 156.For example, the increase in sampling frequency may be based on knowncharacteristics of the waveform of the signal 156, such as the rate thesignal 156 increases during a pulse width period 158.

Furthermore, in some embodiments, the sampling frequency in the otherportions 174 of the signal 156 may have a sampling frequency that isless than the periodic sampling frequency used in typical pulseoximeters and/or less than the Nyquist sampling frequency. For example,a typical pulse oximeter may sample a signal output from a detector atabout 1211 Hz periodically throughout a detection time. Such a samplingfrequency may produce an 8 bit digital signal from the sampled analogsignal 156. In the present techniques, a pulse oximeter 100 may sample asignal 156 at a higher sampling frequency than 1211 Hz, such as at 4844Hz (i.e., four times the standard 1211 Hz), when the signal 156 is atthe relevant portion 172. The digitized signal may have greaterresolution and/or increased accuracy, and may be greater than an 8 bitsignal (e.g., 16 bit or 24 bit). The pulse oximeter 100 may also samplethe signal 156 at a lower sampling frequency than 4844 Hz (e.g., lessthan 1211 Hz or not at all), during a less relevant portion 174 of thesignal 156 to save on processing power. More efficient sampling may alsohelp in limiting the size of a digitized signal, such that a smallerdigitized signal may still contain more samples of relevant portions ofthe analog signal.

A red pulse width period 158 is used in FIG. 5, as well as in FIGS. 6and 7 below as an example. The present sampling techniques may apply tovarious portions of a light intensity signal, including an IR pulsewidth period 160 (as in FIG. 3), or a pulse width period correspondingto any other emitted and/or detected light. Further, the sampling methodused in portions of the signal 156 outside of the pulse width period158, such as dark periods 162 and 164 (FIG. 3) may be periodic (notshown). The sampling density may also be lower than the sampling densityused during the relevant portion 172 of the signal, as a high samplingdensity during dark periods 162 and 164 may not be as useful a highsampling density during the relevant portion 172 for producing anaccurate digitized sample. The dark periods 162 and 164 may still besampled so that the DC content, such as any noise or interferencesresulting from ambient light, may be removed before computingphysiological data from the digitized signal.

The graph 190 of FIG. 6 depicts another embodiment of sampling at ahigher sampling density during a comparatively relevant portion 172 of alight intensity signal 156, in accordance with the present techniques.In one embodiment, the pulse oximeter 100 may begin sampling at thehigher sampling density at a particular start time 192 in each pulsewidth period 158. The start time 192 may be based on known informationabout the signal 156 or known information about a typical waveform of asignal 156 in the pulse width period 158. For example, the pulseoximeter 100 may be programmed to start sampling at a start time 192where the signal amplitude typically nears a maximum amplitude.

In one or more embodiments, the relevant period 172 may be sampled at ahigher sampling density while an earlier portion 194 of the signal 156in the pulse width period 158 before the start time 192 may not besampled. For example, the earlier portion 194 may not be sampled, andonce the signal 156 reaches the start time 192 of the pulse width period158, the pulse oximeter 100 may begin to exponentially sample therelevant portion 172. In some embodiments, the earlier portion 194 maynot be sampled, while the relevant portion 172 may be sampled at orabove the Nyquist frequency. In another embodiment, the relevant portion172 may be sampled at a linearly increasing frequency. In yet anotherembodiment, the earlier portion 194 may not be sampled, while therelevant portion 172 is sampled at a frequency that is substantiallyproportional to the amplitude increase according to a typical waveformof the signal 156.

Alternatively, the earlier portion 194 may be sampled periodically, at asampling density less than the sampling density during the relevantportion 172. In each of the above examples of increased sampling duringthe relevant portion 172, the earlier portion 194 may be either notsampled, or sampled periodically at a lower sampling density than thatof the relevant portion 172. For example, in one embodiment, the earlierportion 194 may be sampled periodically at or below the Nyquistfrequency, and after the start time 192, the relevant portion 172 may besampled at or above the Nyquist frequency.

Sampling at an increased frequency starting from a start time 192 (e.g.,sampling exponentially, sampling at or above the Nyquist frequency,sampling at a linearly increasing frequency, sampling at aproportionally increasing frequency, etc. after the start time 192) mayprovide a more accurate digital signal. In embodiments, sampling at anincreased frequency after the start time 192 may increase theprobability of sampling the signal 156 at a maximum amplitude in eachpulse width period 158. The pulse oximeter 100 may also save onprocessing power by reducing (or eliminating, in some embodiments)sampling during less relevant portions of the signal 156, such theearlier portion 194. Further, the dark periods 162 may still be sampledperiodically (not shown) for purposes such as filtering and/or removingnoise such as ambient light contribution to the light intensity signal156.

In another embodiment, a sampling method of the present techniques maybe based on the waveform and the Nyquist frequency of the signal 156. Asdepicted in the graph 200 of FIG. 7, the signal 156 may be sampled atdifferent frequencies throughout a pulse width period 158. For example,the pulse oximeter 100 may use a sampling frequency which may be lowerthan the Nyquist frequency of the signal 156, until the start time 202,which may be based on a time during a pulse width period 158 when asignal 156 typically nears a maximum amplitude. The pulse oximeter 100may then sample above the Nyquist frequency (e.g., oversample) after thestart time 202. Thus, the relevant portion 172 of the signal 156 duringthe pulse width period 158 may be oversampled to increase theprobability of producing a more accurate digitized signal. In otherembodiments, the portion of the signal 156 in the pulse width period 158prior to a start time 202 (i.e., the earlier portion 174) may not besampled at all to further save on processing power. In some embodiments,the dark periods 162 of the signal 156 may be sampled periodically, forexample, at a frequency lower than the Nyquist frequency. Using a lowersampling density during portions of the signal 156 where a high samplingfrequency may be less beneficial may save on processing power of thepulse oximeter 100. Meanwhile, using a high sampling density duringrelevant portions 172 of the signal 156 may improve a digitized signal,and may improve the accuracy of the physiological characteristicscalculated from the digitized signal.

While the disclosure may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the embodiments provided hereinare not intended to be limited to the particular forms disclosed.Indeed, the disclosed embodiments may not only be applied tomeasurements of blood oxygen saturation, but these techniques may alsobe utilized for the measurement and/or analysis of other bloodconstituents. For example, using the same, different, or additionalwavelengths, the present techniques may be utilized for the measurementand/or analysis of carboxyhemoglobin, methemoglobin, total hemoglobin,fractional hemoglobin, intravascular dyes, and/or water content. Rather,the various embodiments may cover all modifications, equivalents, andalternatives falling within the spirit and scope of the disclosure asdefined by the following appended claims.

What is claimed is:
 1. A method of processing a signal from a pulseoximetry sensor, comprising: using a pulse oximeter, increasing asampling rate of the signal substantially in proportion to an amplitudeof the signal during a sampling period, wherein the sampling rate isbelow a Nyquist rate during a first portion of the sampling period andat or above the Nyquist rate during the second portion of the samplingperiod.
 2. The method of claim 1, comprising using a processor tocalculate an oxygen saturation value based at least in part on thesampled signal.
 3. The method, as set forth in claim 1, whereinincreasing the sampling rate comprises oversampling the signal toward anend of the sampling period.
 4. The method, as set forth in claim 1,wherein the signal comprises a red pulse width period, an infrared pulsewidth period, a dark period, or a combination thereof.
 5. The method, asset forth in claim 1, wherein the sampling period comprises a red pulsewidth period or an infrared pulse width period.
 6. The method, as setforth in claim 5, wherein both the first and the second portions of thesampling period comprise a portion of the red pulse width period or theinfrared pulse width period, and wherein the sampling rate is constantduring the first portion of the sampling period, and wherein thesampling rate is increasing during the second portion of the samplingperiod.
 7. The method, as set forth in claim 1, wherein the samplingperiod comprises a red pulse width period and a dark period, and whereinthe first portion of the sampling period comprises the dark period, andwherein the second portion of the sampling period comprises a maximumamplitude of the red pulse width period.
 8. The method of claim 1,wherein the sampling rate is constant during the first portion of thesampling period, and wherein the sampling rate is increasing during thesecond portion of the sampling period.
 9. The method of claim 8, whereinthe sampling period comprises a red pulse width period and a darkperiod, and wherein the first portion of the sampling period comprisesthe dark period, and wherein the second portion of the sampling periodcomprises at least a portion of the red pulse width period.
 10. Themethod of claim 8, wherein the sampling period comprises a red pulsewidth period and a dark period, and wherein the first portion of thesampling period comprises the dark period and a first portion of the redpulse width period, and wherein the second portion of the samplingperiod comprises a second portion of the red pulse width period.
 11. Amethod of processing a signal from a pulse oximetry sensor, the methodcomprising: using a pulse oximeter, sampling the signal at a relativelylow sampling rate during a less relevant period of the signal and at arelatively high sampling rate during a more relevant period of thesignal, wherein the relatively low rate is at or below a Nyquist rate ofthe signal, and wherein the relatively high rate is at or above theNyquist rate of the signal, and wherein the more relevant period of thesignal comprises a period where the signal is at a maximum amplitude ofthe signal and wherein the less relevant period of the signal comprisesa period before the signal reaches the maximum amplitude of the signal.12. The method, as set forth in claim 11, wherein sampling the signalcomprises increasing the sampling rate substantially in proportion to anamplitude of the signal.
 13. The method, as set forth in claim 11,wherein sampling the signal comprises increasing the sampling ratesubstantially linearly during a sampling period.
 14. The method, as setforth in claim 11, wherein sampling the signal comprises increasing thesampling rate substantially exponentially during a sampling period. 15.The method of claim 11, wherein the less relevant period of the signalcomprises at least a portion of a dark period, and wherein the morerelevant period of the signal comprises at least a portion of a redpulse width period or an infrared pulse width period.
 16. The method ofclaim 11, wherein the less relevant period of the signal comprises afirst portion of a red pulse width period or an infrared pulse widthperiod, and wherein the more relevant period of the signal comprises asecond portion of the red pulse width period or the infrared pulse widthperiod, and wherein the first portion is prior to the second portion.17. A method of processing a signal from a sensor, comprising: using amedical device, sampling the signal at a first sampling rate and at apredetermined time, sampling the signal at a second sampling rategreater than the first sampling rate, wherein the predetermined time isdetermined by a processor of the medical device based at least in partupon a known characteristic of the signal.
 18. The method of claim 17,wherein the known characteristic of the signal comprises a time when thesignal amplitude typically approaches a maximum amplitude.
 19. Themethod of claim 17, wherein sampling the signal comprises increasing thesampling rate substantially in proportion to an amplitude of the signal.20. The method of claim 17, wherein the first sampling rate is below aNyquist rate, and wherein the second sampling rate is at or above theNyquist rate.